The inclusion of general guidance in BS7671, relating to the requirements to select the appropriate Type of RCD, based on the characteristics of the load, seems to have comes as a bit of shock (excuse the pun), for some electricians.

You might ask "Why are Type AC RCDs still referred to in the 18th Edition of BS7671, when most installations should be using Type A devices as a minimum requirement?"

Type AC RCDs are in clause 531.3.3 of BS7671 2018, because they are still in EN 60364-5-53 2015. The UK have just adopted this line from the EN standard.

This is explained in Chapter 12, 13 and Chapter 30. The Electrician is responsible, not the Technical committee, to make the decision on which device is appropriate for the installation.

Put simply, if you are responsible for the design, selection and or testing of the installation:

The revised RCD guidance in 531.3.3 is precise. The appropriate Type of RCD must be selected, based on the characteristics of the residual current and the equipment that is likely to be connected to the RCD protected circuit.

The term general purpose relating to Type AC devices should not be taken out of context i.e. it is the most basic RCD, suitable for detecting a simple full sine wave residual current at 50Hz.

The method of installation requires 30mA RCD protection – e.g. Regulation 415.1 & 522.6.202. Dedicated circuit feeding an immersion heater, conventional oven or cooker, with resistance heating elements.

The initial verification required under 641.1 and 301.1 (ii), covers the requirements for the verification that electrical equipment (any appliance supplied or connected, as part of the existing fixtures and fittings) is compatible with other electrical equipment such as RCDs installed to provide additional protection e.g. for a domestic installation, document the appliance RCD requirements with the justification for the consumer unit specification.

This information should be held on record, with other design criteria and the Installation Test Certificate for the premises. This will be required in the event of claims for negligence (Public liability insurance).

Type AC RCDs subjected to pulsed waveforms will not trip within the safe limits, required for 30mA protection. Appliances containing diodes, produce pulsed residual currents – see Fig 1. Diodes became so common in domestic equipment some years ago, that many countries banned the "general use" of Type AC RCDs e.g. From 1985, VOE Regulations specified the use of Type A RCDs in all domestic installations as a minimum requirement.

Circuit feeding a single-phase micro-wave oven with inverter-controlled output to the magnetron. A fault on the output side of the bridge rectifier will produce a pulsed fault current (If fig 1), generated at the supply frequency. For applications using single phase inverters for speed control (washing machines, GSHPs etc ), the design characteristics of a single-phase inverter will determine if it is safe to use Type A RCDs upstream.

The method of installation relating to protective earthing will depend on the operational leakage current (protective conductor current) – see 543.7. The use of a 30mA RCD may be required for several reasons; cable installation, location / application area for the pump etc. Take note of 531.3.2 Operational leakage current (protective conductor current) for the circuit should not exceed 30% of the RCD sensitivity.

Due to the variation in inverter speed control designs, only the “Manufacture” of the appliance can verify which Type of RCD to use with their equipment. Check installation manuals or contact the Manufacture for advice.

Single-phase inverter designs, applied in washing machines and handheld appliances, containing variable speed features, may produce mixed frequency leakage currents. Appliances containing inverters and motors, can be used with Type A RCDs, if:

Type A RCDs are not designed to detect smooth dc residual currents. They will still trip within safe limits with < 6 mA smooth dc superimposed on a 50Hz pulsed residual current.

* Type A RCDs are not designed to provide electric shock protection in circuits containing mixed frequency leakage currents, associated with faults on the output side of the inverter see Fig 2 fault current If2

* High frequency current components can saturate the RCD, preventing detection of 50Hz residual currents.

A single-phase appliance, that produces additional frequency components superimposed on the 50Hz sine wave. e.g. PWM speed control for a pump – see Fig 3. To provide additional protection, a 30mA RCD must continue to operate within the defined tripping requirements of Regulation 643.8. Type F RCDs meet EN62423** for composite residual currents – see Regulation 531.3.3 (iii)

For general installations, domestic electricians may not be equipped to check power quality frequency components, generated by particular appliances. For new installations the electrician relies on the information provided by the appliance manufacturer, the advice given in BS7671 and technical help from their Trade Associations.

For existing installations, problems associated with the following may indicate that Type F is required in place of Type A:

Appliances containing inverters and motors, can be used with Type F RCDs, if:

Type F RCDs are not designed to detect smooth dc residual currents. They will still trip within safe limits with < 10 mA smooth dc superimposed on a 50Hz pulsed residual current.

*** Type F RCDs cannot provide protection in circuits where there is a requirement to detect smooth DC residual currents, or if the motor frequency can be reduced to zero or the inverter switching frequency is >1kHz. If any of the above conditions apply, Type B RCDs must be used.

For single phase inverters used in motor speed control, the characteristics of the equipment will determine if Type A, F or B is required, to meet Regulation 531.3.3 For 3 phase inverters which do not incorporate isolating transformers, Type B RCDs must be used to meet the requirements of Regulation 531.3.3 For additional information.

Please refer to www.Doepke.co.uk

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